The present invention relates to an artificial kidney.
It is known that kidneys of living organisms are important excretory organs whose main functions are to remove both the superfluous water and the biologically useless as well as toxic materials accumulating in the blood. The human kidney is composed of 1-1.5 million microscopic subunits called nephrons. Each nephron in turn has a complex structure with two main parts: the glomerulas and the tubules. In the glomerulas the blood plasma is filtrated from blood capillary vessels through porous walls of these vessels. The filtrate drains into the tube system forming the tubules where the major part of water and plasma components are readsorbed into the efferent blood vessels. The remaining liquid containing all biologically useless materials in a high concentration is the urine that passes through the ureter to the urinary bladder. Approximately 180 liters per day of blood passes through the kidneys producing only approximately 1.5 liters of urine.
Urine contains hundreds of organic compounds, most important of them being the protein digestion and metabolism products, urea, creatinine, uric acid and some others. When kidneys cannot operate properly, useless and/or toxic materials accumulate in blood and other physiological fluids and lead to death within 10-12 days. Replacing the ill kidney with a healthy one by transplantation stimulates the rejection mechanisms of the living body against the foreign organ, unless the donor is a close relative. Therefore, anti-rejection drugs must be given to the recipient patient, which always have some harmful side effects. Therefore, a transplanted kidney cannot generally be expected to keep functioning for more than five years.
The only alternative way of removing excess water and biologically useless low-molecular-weight organic compounds from the organism is external hemodialysis. By letting blood or blood plasma to equilibrate with a special dialysate aqueous solution through a semi-permeable polymeric membrane, this technique allows both the excess water and small molecules to migrate down the concentration gradient from the blood into the dialysate fluid. The pores of the membrane are usually chosen small enough in order to prevent larger molecules such as proteins from diffusion. For smaller molecules, the sole driving force for diffusion is the concentration gradient, since these molecules after having passes the membrane are constantly removed by a flow of fresh dialysate liquid. This mechanism holds for the water removal as well, since the dialysate fluid additionally contains salt in high concentrations, which diminishes the activity of water or which is basically the same, causes a difference in the osmotic pressures on the opposite sides of the membrane.
Hemodialysis is a slow process, which keeps the patient connected to the dialysis machine for several hours. This procedure has to be repeated three or four times a week. Besides the high consumption of the physiological dialysate fluid (about 120 liters), the technique is expensive as well as unpleasant and inconvenient for the patient. The patient will feel unwell both before and after dialysis. Before dialysis the waste products build up in the body, and after dialysis there is a dramatic distortion of the balance of chemical equilibria and processes in the body due to removal of the whole pool of molecules of the molecular weight of less than 500 dalton. Among these molecules are all amino acids, nucleotides, mineral ions and many other useful components. To minimize the loss of essential components during dialysis, the physiological dialysate fluid is doped with Na, K, Cl, Ca, Mg, acetate, bicarbonate and glucose. It would be too expensive, if possible at all, to add other components to the fluid. This drawback of the technique results from the unselectivity of the diffusion process through the polymeric membrane: the latter does not distinguish between useful and useless molecules. This unavoidable harmful effect would be expressed much less, if the removal of small molecules would be slow and constant instead of being made three or four times a week with a total clearance.
Two U.S. Pat. Nos. 5,092,886 and 4,769,037 disclose implantable artificial kidneys which should mimic the processes of filtration and partial readsorption of useful components that are characteristic for the living kidneys. The first patent suggests an extremely complicated system of tubes embedded in one another with the inner tubes having permeable porous walls and the outer tubes having impermeable walls. After arriving at a certain cross-over point, the outer impermeable tube should enter the interior of the previously inner permeable tube, with the permeability of the walls of the tubes inverting. This should provide the possibility for the reversal of filtration, i.e. give rise to readsorption of useful components.
The physico-chemical feasibility of these procedures raises serious doubts. Contrary to permeation through a biological membrane, which is an active specific transportation of selected molecules, diffusion through the polymeric membrane is nothing more than a simple size-restricted down-gradient flow of molecules, which is totally unspecific with respect to biological value of the compound. Therefore, no selective removal of waste products and no selective readsorption of useful compounds can be organized by ultrafiltration, regardless of the fact that the diffusion proceeds from the inner tube into the outer one or vice versa. Small molecules always display the tendency of appearing at equal concentrations on both sides of a polymeric ultrafiltration membrane. The patented project does not even try to consider gradients of concentrations as a driving force for diffusion of compounds, neither pressure differences as the driving force for the overall flow of fluids from one chamber into another.
The same disadvantages are characteristic of the other U.S. Pat. No. 4,769,037 which suggests placing sponge-like polymeric materials between extremely complicated hollow panels made of semipermeable membranes.